A transconductance amplifier receives an input voltage and outputs a current having some defined relationship to the input voltage. The transconductance (gm) of the amplifier is typically constant for a wide range of input voltages, but a nonlinear transconductance is desirable for some applications. The transconductance is given as gm=ΔIout/ΔVin and may be variable for different ranges of ΔVin.
The output of a transconductance amplifier may be used to charge and discharge a capacitor, control a MOSFET or bipolar transistor, or used for other purposes. Rapidly charging/discharging high capacitance loads, or driving large MOSFETs, may require a transconductance amplifier with a higher gm.
One example of the usefulness of a transconductance amplifier having a nonlinear gm is in a feedback circuit such as used in a voltage regulator. The amplifier may receive a reference voltage and the regulator's divided output voltage. If the two voltages match (during steady state operation), the current output of the amplifier is substantially zero and the output voltage of the regulator is not changed. When the divided output voltage of the regulator becomes greater than or less than the reference voltage, such as due to a change in the load, the non-zero output current from the amplifier causes the output voltage of the regulator to go down or up to again match the divided output voltage to the reference voltage. Since there are transient signals and delays involved in the operation of such a feedback circuit, stability is a concern. By the amplifier having a low gm for small input voltage differences, the regulator has added stability and, by the amplifier having a high gm for large input voltage differences, the regulator can quickly react to changing loads.
Such nonlinear gm transconductance amplifiers have many other uses.
The gm can be asymmetric, such as where a positive voltage differential causes the amplifier to have a particular gm and where a negative voltage differential causes the amplifier to have a different gm. Such an asymmetric gm may be where the amplifier is driving a device to emulate a diode.
FIG. 1 illustrates the current output of a transconductance amplifier having stepped gms and an asymmetric output. When there is a negative voltage differential at the amplifier's input, the amplifier has a high gm, labeled H. The input may have an offset voltage, causing the amplifier to output substantially zero current during the offset range, labeled O. For a small positive voltage differential above the offset, the amplifier is configured to have a small gm, labeled S. For a larger positive voltage differential, the amplifier is configured to have a medium gm, labeled M. For an even larger positive voltage differential, the amplifier is configured to have a high gm, labeled H.
Multiple transconductance amplifiers may be interconnected to achieve the desired characteristics of FIG. 1.
FIG. 2 illustrates one prior art technique for varying gms of a composite transconductance amplifier, and FIG. 3 is a current vs. voltage curve obtained by the amplifier of FIG. 2.
The individual transconductance amplifiers are labeled as 10, 11, 12, 13, and 14 and have respective transconductances of gm, gm1, gm2, gm3, and gm4. The gms may be the same or different. The amplifiers 10-14 operate in different combinations to perform as a single transconductance amplifier 16 having tailored characteristics. Different voltage offsets V1-V4 are shown coupled to the inverting inputs of the amplifiers. The polarities of the voltage offsets are identified. The diodes D1-D4 are not part of the circuit but just convey the different directions of current output by the amplifiers 10-14. The composite amplifier 16 is shown driving a capacitive load 20.
FIG. 3 shows the current vs. input voltage waveform for the amplifier 16 of FIG. 2 as the input voltage Vin is swept from a negative voltage to a positive voltage. The waveform is symmetric about zero volts. The offset voltage levels are identified as V1-V4. The offset voltages determine when each amplifier 11-14 contributes to the output current. The amplifier 10 is always contributing to the output current and operates through the full voltage range. The amplifier 11 contributes current when a positive input voltage differential exceeds V1, and its gm is combined with that of the amplifier 10, shown as the medium gm M1 in FIG. 3. The amplifier 12 contributes current when the positive input voltage differential exceeds V2, and its gm is combined with that of the amplifiers 10 and 11, shown as the high gm H1. For a negative input voltage, the amplifier 13 contributes current when the negative input voltage differential exceeds V3, and its gm is combined with that of the amplifier 10, shown as the medium gm M2. The amplifier 14 contributes current when the negative input voltage differential exceeds V4, and its gm is combined with that of the amplifiers 10 and 13, shown as the high gm H2. The composite amplifier 16 could have had an asymmetric output by changing the gms of the individual amplifiers or the offsets.
Drawbacks with the design of FIG. 2 include the gms having discrete levels and each individual amplifier always drawing a quiescent current. More amplifiers can be added to increase the gm levels but this adds to the quiescent current, complexity, and cost.
What is needed is an improved transconductance amplifier with a nonlinear gm, where the quiescent current of the amplifier is much lower than that of the prior art design.